This invention relates to a composition and method for removing impermeable layers created for fluid loss control in a subterranean formation. More particularly it relates to the use of a fluid containing a delayed solid acid material that can either remove drilling mud filtercakes or create a self-destructing filtercake in subterranean formations that require fluid loss control.
During drilling of a well, a thin layer of impermeable material is deposited on the reservoir rock by the drilling fluid (or mud). This thin layer of material is called a filtercake and aids in controlling drilling fluid leak-off into the formation and restricts the inflow of reservoir fluids into the well during completion. If the filtercake that is created during the drilling process is not removed prior to or during completion of the well, problems may occur when the well is put on production. These may include completion equipment failures, such as erosion and plugging of the equipment, and impaired reservoir productivity, which may be in the form of early water production or water coning.
The major components typically found in conventional drilling mud filtercake include such materials as polymers, carbonates and other inorganic salts, and clays. Removal of the mud filtercake can be accomplished through mechanical means (scrapping, jetting, underreaming, etc). Conventional chemical treatments for removing filtercake include pumping aqueous solutions with an oxidizer (such as persulfate), inorganic acids (such as HCl), organic acids (such as acetic or formic acids), chelating agents (such as EDTA), enzymes or combinations of these. Generally, the oxidizer or enzyme digests the polymer layer in the filtercake and the acids dissolve the carbonate portion in the filtercake.
There are several problems that exist in conventional filtercake removal. The acids used tend to react very quickly with carbonate and “wormholes” are readily formed where most of the acid will funnel off through these small openings into the reservoir and leave most of the zone untreated. Oxidizers are very corrosive and reactive. They also must be pumped as a separate stage, which causes operational complexity and extra cost. Additionally, the lifetime of an oxidizer at higher temperatures may be only a few seconds. Enzyme breakers are extremely sensitive to pH, temperature, and ionic strength. They are not effective in breaking polymers in acidic solutions and will lose their activity at higher temperatures. Chelants are weak acids and poor dissolvers of carbonate compared to other organic acids, such as acetic and formic acid. Catalysts and activators require a second step in the completion process that causes additional operational costs.
Depending on the composition of the formation (such as quicksand or unconsolidated sands, partially consolidated sands, or friable sands), sand control operations such as gravel packing are commonly implemented in order to stabilize a wellbore after drilling. In a gravel packing operation, a screen is placed in the wellbore and then gravel of a specific size is placed in the annulus between the screen and the reservoir to stop sand movement and sand production from the reservoir. The installation of a gravel pack typically entails the use of a carrier fluid (such as brine).
There are many oilfield applications that require the use of fluid loss control agents in the near well-bore region, within the formation itself or against sand control screens and gravel packs. Some of these applications are in cased and perforated wellbores, while others are in open holes.
In cased hole applications, the need for fluid loss control may occur immediately after perforating. In such cases, the most common practice has been to perforate with clean fluids (such as completion brines) and spot a fluid loss control pill to control losses into the formation while the perforating string is pulled out of the hole and either the production string or the sand control screens are run into the wellbore. In the former case, once the tubing is installed, the well is put on production. Presence of a low permeability filtercake formed by the fluid loss control pill often causes reduction in well productivity, unless a filtercake removal treatment is performed. Such treatments typically involve pumping acids into the wellbore. In the latter case, once the sand control screens are run in hole, often gravel packing or frac-packing is performed. The presence of a low permeability filtercake formed by the fluid loss control pill spotted after perforating can cause either unpacked or incompletely packed perforations, both of which result in productivity impairment. In these cases, often an acid treatment is performed prior to pumping a gravel pack or frac-pack treatment to ensure a high permeability gravel/proppant pack is placed into the perforations. Fluid loss is often experienced after frac-packing as well. Although the common practice for fluid loss control after frac packing is typically to use mechanical valves that isolate the zone of interest from the hydrostatic column above it, sometimes these valves fail, necessitating a fluid loss control pill to be spotted inside the sand control screens in order to provide fluid loss control while the sand control service tool is pulled out of the wellbore and the production tubing is run in hole. Thus, it would be desirable to have a fluid loss control pill that can form a filtercake, which will control losses for the desired time period (e.g., the time it takes to pull the perforating string out and run either the production tubing or the sand control string in to the wellbore, or pull the sand control service tool out and run the production tubing in) and self destruct when needed (e.g., when ready to produce or when the sand control screens are installed).
In open hole applications, fluid loss control pills may be needed immediately after sand control in cases where a mechanical valve has not been installed or the valve failed to operate. In such cases, the fluid loss control must be achieved against the screens and gravel.
In open hole completions, the low permeability filtercake is necessary during drilling, but can cause productivity impairment during production. In fact, in sand control completions, for example during gravel packing, particularly with low viscosity fluids such as brines (i.e., water packing), filtercake is required in order to gravel pack a well successfully, and thus it must be removed after gravel packing in order to maximize production rates as well as to provide uniform influx of hydrocarbons into the wellbore. Similarly, in stand alone screen or expandable screen applications (which do not involve gravel packing), filtercake is necessary for wellbore stability until the screens are installed, but needs to be removed prior to production. Filtercake removal in open hole completions has traditionally been done through spotting and/or circulation of acids. Although acids are effective in filtercake removal, assuring their contact with the filtercake across a long open hole section is not easily accomplished. This is because the reaction between the acid and the filtercake is very rapid, resulting in increased permeability in the sections where the acid contacts the filtercake first, and thus loss of all the acid into that section of the wellbore. The net result in this case is clean and unclean sections along the wellbore. This is undesirable since it results in reduced production rates and/or non-uniform influx, the latter causing premature water or gas breakthrough. Alternatively, enzyme or oxidizer soaks (to hydrolyze polymeric components of the filtercake) are performed, followed by an acid treatment. This process is also ineffective, since the reaction of the acid with carbonate bridging agents in the absence of coating with polymeric components (as it would be after the enzyme or oxidizer soak) is much faster than the reaction of acid with all components of the filtercake intact, causing the same problems. In addition, acids are corrosive to sand control screens.
Other alternatives include combining chelating agent solutions, which provide much slower reaction rates with much lower corrosion rates. Although enzyme and chelating agent solutions, or chelating agent solutions alone, have been effectively used in open hole completions, in longer wells, and particularly at higher temperatures, even they may react relatively quickly in long open hole completions, particularly in sand control applications. In such cases, after sand control treatment is performed, the wash pipe must be pulled out of the screens, activating a fluid loss control valve to isolate the formation from the hydrostatic column above.
Thus, it is desirable not to remove the filtercake until after the wash pipe is pulled out of the screen and the fluid loss valve is closed. In such cases, it is desirable to have a filtercake cleanup solution which is not reactive until after some time (e.g., until after the wash-pipe is pulled and the formation is isolated).
There is therefore a need to provide improvements in compositions and methods for filtercake removal and for providing effective and readily reversible fluid loss pills or treatments.